Chief isn't a traitor?- spoilers

so, after a marketing campaign that put huge emphasis on chief betraying the UNSC in halo 5, I can help but notice that throughout the entire campaign chief at no point actually comits treason against the UNSC.

the closest thing he did was go awol, which is illegal, but not treason. It would have been treason had he joined the covenant or the innies, but unless I missed something major, he didn’t do any of that.

which leads to my question about hunt the truth. Why did ONI decide to “kill” him. He didn’t do anything that they couldn’t have covered up by simply not saying anything. It’s not like chief going awol would have been public knowledge anyway, and also NOT TREASON.

so what do you think?